Chapter 26 – The Vertebrates
Animal Evolution: Every animal is a combination of traits, some which are conserved from
remote ancestors and others that are unique to its branch of the family tree.
All chordates share four features at some point of their development:
Notochord supports body
Nervous system develops from dorsal nerve cord
Embryos have pharynx with slits
Embryos have tail that extends past anus
Urochordata: Salps and tunicates
Cephalochordata: Lancelets
Craniates: Fishes, amphibians, reptiles, birds, mammals. Cartilaginous or bony chamber
encloses and protects brain
Trends in the Evolution of Vertebrates
Shift from notochord to vertebral column
Nerve cord expanded into brain
Evolution of jaws
Paired fins evolved, gave rise to limbs
Gills evolved, gave rise to lungs
Adaptations to life on land - reptiles
Tough, scaly skin
Internal fertilization
2 kidneys
Amniote eggs
Adaptive Radiation: The early reptiles were very successful, giving rise to modern reptiles,
birds and mammals.
Amphibians
Animals in the class Amphibia were the first that were able to spend at least a portion of their
life on land. The first amphibians were probably primarily aquatic, making occasional trips on
land to escape predation or in search of food (especially the insects that were undergoing their
own adaptive radiation explosion).
Its adaptations to living on land include a denser skeleton and appendages modified for moving
on land and supporting its weight, improved lungs, more complex circulatory system (3 chambered heart), and improved hearing & vision and water conservation adaptations (e.g., more
efficient kidneys).
What is the geographic distribution of amphibians? What habitats do they inhabit?
Temperate, sub-tropical and tropical latitudes of the world. Amphibians are often associated with
streams or wetlands, including seasonal wetlands. Access to moist conditions or fresh standing
or running water is important for amphibians.
How do amphibians differ from fish? Name at least three adaptations that allow
amphibians to live on land.
Amphibians have lungs to breathe (pulmonary respiration) as well as cutaneous respirations. Fish
generally lack lungs, and instead use gills for respiration.
Many amphibians undergo complex life cycles while fish do not. Note that not all species of
amphibians have a distinct larval stage (like a tadpole). These are known as “direct developers”.
Amphibians have ears, eyes (including eyelids) that allow then to survive on land. They also can
vocalize to communicate on land.
Even though amphibians are terrestrial, they tend to be associated with water and/or moist
habitats. Why?
Water is essential to successful reproduction (e.g., some species of tropical tree frogs lay their
eggs in rainwater captured in bromeliads in the forest canopy). The eggs are particularly
vulnerable to desiccation. Larval forms of many amphibians live in aquatic habitats, even if the
adult form is partially or completely terrestrial.
In addition, amphibians can easily dehydrate due to their porous skin.
Describe amphibian reproduction, including where fertilization takes place (internal or
external), the type of eggs laid, and where egg-laying takes place.
Amphibians reproduce sexually. Fertilization – the process of sperm uniting with egg occurs
externally in most species of frogs (though the tailed frog in the Pacific Northwest is an
important exception because the male deposits the sperm inside of the female). Females lay the
eggs and the males deposit sperm in the same location.
Some salamanders have internal fertilization in that males release packets of sperm, called
spermataphores. Females then pick up the packet and inserts it into their cloaca where
fertilization occurs.
Most amphibians demonstrate no parental care. However, as usual, there are exceptions and
some do guard the eggs and even feed young (e.g., the strawberry poison dart frog).
Compare adaptations to terrestrial life and reproduction with that of the bryophytes (e.g.
mosses) and seedless vascular plants (e.g. ferns).
Bryophytes, like many amphibians are terrestrial, but are closely associated with water. Most
amphibians lay their eggs in or near water and have external fertilization that is facilitated by
water. Bryophytes, also require water to facilitate the movement of the flagellated sperm to
fertilize the egg (usually rain or dew).
Reptiles
The class Reptilia includes turtles, snakes lizards, alligators and Komodo dragons. All of them
have lungs to breathe on land and dry, scaly skin that does not need to be kept wet.
What is the geographic distribution of reptiles? What habitats do they inhabit? Reptiles
occur primarily in temperate and tropical latitudes, a couple of species occur near the Arctic
region. Reptiles inhabit nearly all terrestrial habitats as well as marine and fresh water habitats.
Describe reptile reproduction, including where fertilization takes place (internal or
external), the type of eggs laid, and where egg-laying takes place.
Reptiles primarily reproduce sexually. Fertilization is internal, thus making water unnecessary
for the sperm to meet the egg. Most reptiles lay eggs, however a few snakes are live bearers.
What is an amniote egg? How is production of an amniote egg an advantage over
amphibian eggs?
An amniote egg is one that has a protective covering that keeps it from desiccating. In addition, it
contains three membranes used for protection, gas exchange and nourishment for the developing
embryo. Reptiles, unlike amphibians, do not require water for their eggs allowing them to utilize
many terrestrial habitats inaccessible to amphibians.
What kind of parental care do reptiles demonstrate?
Most reptiles demonstrate little or no parental care after laying their eggs and young are
completely independent at hatching. However, some reptiles and snakes guard their nests and
crocodiles guard their nests and hatchlings (though they don’t feed them). Incidentally,
crocodiles are the most closely related reptile to birds.
Compare reptilian adaptations to terrestrial life and method of reproduction with that of
the gymnosperms.
It is most useful to compare two characteristics of reptiles and gymnosperms. First, fertilization
is internal in reptiles, negating the need for water to facilitate the movement of sperm (compare
to amphibians). Pollen (containing the gymnosperm sperm) also does not need water and
gymnosperms are wind pollinated. Next, the development of the amniote egg allowed early
reptiles to lay their eggs on land. This parallels the development of the seed in plants, which
protected the developing sporophyte embryo in a protective seed coat.
Birds
The class Aves includes all of the 9,800 recognized bird species in the world (of which only 930
are found in North America and 350 regularly occur in Oregon). Although northern states
generally support fewer species of birds than southerly ones, Oregon ranks fifth behind only
Florida, New Mexico, Texas, and California in terms of numbers of species.
What is the geographic distribution of birds? What habitats do they inhabit?
Birds have a worldwide distribution and occur in both polar regions at least part of the year.
Consequently, they inhabit most terrestrial and marine habitats (including open ocean). Their
ability to fly allowed them to colonize remote oceanic islands inaccessible to other vertebrate
groups. Although some birds spend the majority of their lives at sea (e.g., albatross and
penguins), all birds must come to land to lay their eggs.
Name major adaptations birds evolved for flight.
Important modifications to the skeleton, including the forelimb modifications to support wings.
Overall there are a number of structural modifications (including pneumatization and fusion of
many bones) that allow the skeleton to be simultaneously lightweight and strong.
Large breast (pectoral) muscles and a keeled breastbone for attachment.
Scales modified into lightweight and interlocking flight feathers that provide lift.
Highly developed respiratory system to meet birds’ energetic needs during flight.
Some scientists believe that birds are simply reptiles with feathers. Name 2 characteristics
of birds that support this hypothesis (e.g. how are they similar to reptiles).
Both have scales. Scales are obvious a bird’s feet, and feathers are derived from the same tissues
as scales.
Both lay eggs. The structure of eggs are very similar.
Nucleated red blood cells (FYI – we, like all mammals, do not nuclei in our red blood cells).
So-called transitional species from the fossil record, like Archaeopterix, show characteristics
found in birds and reptiles.
Also, there are a number of skeletal similarities that birds and reptiles have, but mammals do not.
Describe avian reproduction, including where fertilization takes place (internal or
external), the type of eggs laid, and where egg-laying takes place.
Birds reproduce sexually. Fertilization is internal. Birds, like reptiles lay amniote eggs that have
hard shells made of calcium carbonate. Some species do not build nests, while others build
elaborate nests. Importantly, the eggs must be placed where they will be protected from the
elements and predators and can be incubated. Some birds lay their eggs in the nests of other
species (nest parasites).
What kind of parental care do birds demonstrate? While a few species engage in little or no
parental care (e.g., megapodes and nest parasites), the vast majority of birds both incubate eggs
and care for nestlings through at least fledgling (and usually some time beyond fledging).
Mammals
Humans are members of the vertebrate class Mammalia. There are approximately 5000 mammal
species in the world making up only about 0.4% of known animal species.
What is the geographic distribution of mammals? What habitats do they inhabit? Birds
have a worldwide distribution and occur in both polar regions at least part of the year.
Consequently, they inhabit most terrestrial and marine habitats (including open ocean).
Name traits that are unique to the class Mammalia.
They produce milk for their young via mammary glands.
They have hair.
They have unique skeletal features (e.g., three bones in the middle ear, a single bone comprising
the lower jaw).
They have a diaphragm that separates the body cavity into two sections.
There are both placental and non-placental mammals. Briefly describe how reproduction
takes place in the following groups of mammals and give example of species for each.
Mammals reproduce sexually (Dolly the sheep, aside). Fertilization is internal.
1. Placental mammals: Females give birth to live young. The developing fetus remains in the
female and are provided nourishment via the placenta. We are placental mammals.
2. Monotremes: Mammals that lay eggs. Females actively care for eggs and have prolonged
parental care. Duck-billed platypus, and echidnas.
3. Marsupials: marsupials have placentas and give birth to live young. However, the young are
embryonic at birth and do most of their development attached to the female’s teat in a outer
protective pouch. The only marsupial species in North America is the opossum.
What kind of parental care do mammals demonstrate? Female mammals invest a lot of care
into young by first carrying the developing young (marsupials and placental mammals) and then
feeding them via mammary glands (all mammals), as well a protecting and educating young
following birth. In humans, males are important providers of parental care.
Biologists sometimes compare the angiosperms to the placental mammals. On what do
they base this comparison? In placental mammals, the developing embryo is housed within the
female’s uterus and nourished by the placenta. In angiosperms, The developing embryo is
housed within the ovary and in some plants, the endosperm is critical to the development of the
embryo.
All of these comparisons provide a way to think about the evolution of plants and animals and
how particular innovations have contributed to the success of these groups.
Which of the four vertebrate classes rely solely on lungs to breathe (pulmonary
respiration)? Which habitats are these classes likely to be found relative to the one that also
uses cutaneous respiration? Terrestrial amphibians rely heavily on cutaneous respiration as
well as lungs. Reptiles, birds and mammals breathe (that is, the exchange of O2 and CO2) via the
lungs. The relatively waterproof skin of these vertebrate classes allows them to live in
environments that would be far too desiccating for amphibians including very arid areas and
marine habitats.
Which of the four vertebrate classes are ectotherms, which are endotherms? In what way
does this characteristic affect the general behavior and metabolism of each class?
First, distinguish between ectotherm and endotherm. In ectotherms (cold-blooded), core body
temperature is largely a function of the surrounding environment. The animal’s behavior often
plays a critical role because its temperature depends so much on the ambient conditions (this is
why basking is such an important reptile behavior). In endotherms (warm-blooded), the core
body temperature is controlled largely by metabolic activity and physiological regulation of heat
exchange with the environment. Behavior also is important (e.g., taking shelter from extreme
temperatures), but temperature control is primarily physiological.
Ectotherms tend to do better than mammals in nutrient- and oxygen-poor areas because their
metabolic needs are less demanding that endotherms. This also explains why they can go such
long periods without eating. Endotherms have high demands for oxygen, food and water to meet
their energetic needs. So, for example, a foraging mouse uses 20 - 30 times more energy than a
foraging lizard of equal mass. Consequently, endotherms are much more active than ectotherrms
and much more likely to be nocturnal and active in winter.
Which of the four vertebrate classes were found (or likely would be found) in the Polar
Regions? Which ones were not (or unlikely would be)? Why? (think about adaptations).
This is a follow-up question to the one above. Endotherms can be fully functional in extremely
hot and cold environments, virtually everywhere on the planet, as long as there is sufficient
oxygen, food and water. Mammals and birds have a variety of characteristics (fur, feathers,
accumulation of body fat, large size) and behaviors (shivering, social behavior, hibernation, etc.)
that help them maintain their body temperatures in extreme environments. In general, reptiles
and amphibians live in temperate and tropical latitudes (both terrestrial and marine).
Compare and contrast parental care among the four classes. In general, which of the
classes have young that are more likely to survive to independence? Review the life-history
patterns in section 45.5. The investment in parental care by mammals and birds relative to
amphibians and most reptiles means that their young are more likely to survive to adulthood. So,
they have fewer young over their lifetimes, but each one has a higher probability of surviving.